What Does Lower Than Normal CV Mean for HRV?
A lower than normal coefficient of variation (CV) for heart rate variability indicates impaired autonomic nervous system function, specifically reflecting reduced parasympathetic (vagal) tone and elevated sympathetic activity, which is associated with increased cardiovascular risk and total mortality. 1
Physiological Interpretation
Low HRV CV represents a fundamental autonomic imbalance where:
- Diminished parasympathetic activity is the primary characteristic, indicating impaired vagal tone and reduced capacity for the heart to respond flexibly to physiological demands 1
- Sympathetic predominance occurs, creating an imbalance where sympathetic activity is relatively enhanced compared to parasympathetic modulation 1
- Reduced autonomic flexibility means the heart cannot adapt appropriately to environmental changes and internal stressors 2
The autonomic nervous system normally demonstrates that HRV is almost completely due to autonomic input to the sinus node, providing a surrogate for autonomic effects in the ventricle that are important in the pathogenesis of ventricular tachycardia and ventricular fibrillation 3
Clinical Significance and Risk Implications
Low HRV carries substantial prognostic implications for mortality and cardiovascular events:
- Increased total mortality risk: Patients in the lowest tertile for HRV have significantly increased risk of cardiovascular death, with relative mortality risks of 2-3 1, 3
- Arrhythmic events: In heart failure patients, diminished low-frequency power during controlled breathing is associated with a 5-fold increase in arrhythmic mortality 1, 3
- Sudden cardiac death risk: The combination of impaired low-frequency power and more than 86 ventricular premature beats per hour confers a 23% sudden cardiac death risk compared to 3% in those with preserved low-frequency power 1, 3
- Cardiac arrhythmias are often initiated by or occur in patients with enhanced sympathetic and diminished parasympathetic tone, the exact pattern seen with low HRV 1, 3
However, abundant data show that depressed HRV is a better predictor of total mortality rather than specifically arrhythmic mortality, and despite the theoretical pathophysiological link to arrhythmogenesis, current evidence suggests HRV may be a better marker of nonarrhythmic mortality 3
Important Clinical Caveats
The American Heart Association and American College of Cardiology state that short-term HRV use for sudden cardiac death risk stratification is not currently recommended due to limited data linking impaired short-term HRV specifically to sudden death 1, 3
Additional limitations include:
- Poor reproducibility: Short-term HRV is less reproducible in patients with congestive heart failure compared to normal subjects 1, 3
- Marked interindividual variation in the relationship of short-term HRV to parasympathetic effect makes identification of clear normal/abnormal limits difficult in individuals 1, 3
- Context-dependent interpretation: The genesis of HRV in health and disease is not completely understood, and relative roles of heart rate versus HRV as indicators of autonomic activity continue to be debated 3
Modifiable Contributing Factors
Several lifestyle and medical factors can contribute to low HRV and represent targets for intervention:
- Sleep disturbances: Poor sleep patterns are associated with reduced HRV, while addressing sleep disorders can improve autonomic function 1, 2
- Physical inactivity: Sedentary behavior and lack of regular aerobic exercise negatively impact HRV 1, 2
- Substance use: Smoking and excessive alcohol consumption reduce HRV 1, 2
- Cardiovascular risk factors: Uncontrolled hypertension and diabetes contribute to low HRV 1, 2
Regular aerobic exercise, stress reduction practices that activate the parasympathetic nervous system, and respiratory training with slow controlled breathing can enhance HRV by increasing parasympathetic activity 2